1. Field of the Invention
The present invention relates generally to image sensors, and more particularly but not exclusively to those used in x-ray imagers.
2. Description of the Background Art
An x-ray imager may comprise a CMOS (Complementary Metal Oxide Semiconductor) image sensor and a scintillator. The scintillator converts x-rays to visible light, which impinges a photodiode in the image sensor. The image sensor also includes a readout circuit for reading a pixel of image information from the photodiode. A typical image sensor has several photodiodes and readout circuits to generate an entire image. The image may be displayed on a monitor or printed for viewing purposes.
Readout circuits are typically formed on the same substrate as the photodiodes, and thus limit the useful (i.e., light sensitive) area of the photodiodes. Because readout circuits are not designed to convert light to image information, a metal shield may be formed over the readout electronics to make it insensitive to incoming light. Preferably, incoming light generates free electrons only in regions surrounding the photodiode. In contrast to light, x-rays hitting the image sensor may generate large clouds of free electrons everywhere, even underneath the metal shield. This direct conversion of x-rays, which occurs in silicon portions of the image sensor, result in parasitic electrons that may introduce noise in images.